Here are two useful and reliable online tools (no downloading of software is required):
1. To quickly convert YouTube videos to mp3 files -
https://www.y2mate.com/en30/youtube-mp3A couple of points: it is worthwhile having an ad-blocker on your browser; sometimes you may be converting a file and you encounter the message "Video not available" - this is only temporary (simply put that conversion aside, process some others and then return to the reluctant file a little later).
2. To increase or decrease the volume level of audio files -
https://www.mp3louder.com/ 
